The present invention relates to a method of operating a metal forming press in which in a rigid press frame a workpiece is preloaded against a tool by means of a hold-down force, with at least one ram tool applying a ram force in parallel with the hold-down force, and closing cylinder forces being applied opposite to the ram force and the hold-down force, and a water chamber die being formed in the tool, whereby water chamber die forces can act on the workpiece.
A metal forming press of the above-described type is already known from German patent application 195 13 444.
Said metal forming press is inter alia characterized by the decisive advantage that for both internal high-pressure metal forming and external high-pressure metal forming it is possible to lock both the ram tool and the hold-down devices and to apply the necessary further metal-forming forces by means of a multitude of closing cylinders. Said closing cylinders can be controlled in a selective way and can act on the most different portions of the workpiece owing to their arrangement. Large forces can thus be applied at short piston travels of the cylinders.
However, it has been found out in the operation of the above-described metal forming press that the necessary hold-down forces cannot be realized or can only be realized in an inadequate manner for specific workpieces.
It is the object of the present invention to provide a method of the above-mentioned type which is simple and can be carried out easily and permits the application of exact hold-down forces and is also applicable to the most different workpieces.
According to the invention this object is achieved by the features of the main claim; the subclaims show further advantageous developments of the invention.
Thus, according to the invention, the disturbing force resulting from the application of the ram force is determined and compensated by a change in the closing cylinder forces.
The method according to the invention is characterized by a number of considerable advantages.
It is possible with the procedure according to the invention to compensate the disturbing force which occurs in the whole system due to the application of the ram force. It has been found that the closing cylinders react at least in part like elastic elements and deform upon application of a ram force and/or a water chamber die force. This deformation, in turn, together with the elasticity of the tool, has the effect that the hold-down forces are changing. These are either higher or lower, depending on the corresponding deformation. As a consequence, the metal forming parameters change, resulting in flaws (excessively strong or weak feed flow of the material of the workpiece).
The above-described problems are avoided by the method of the invention.
In sheet-like workpieces the hold-down force controls the sheet movement and produces a sufficient surface pressure on the tool to seal the water chamber die at the pressures arising. The hold-down force is adapted to the necessary water chamber die pressure, ideally in response to the ram travel, and varies over the whole flange area of the workpiece to compensate for locally different metal forming degrees.
In a metal forming press of the above-described type the pressure prevailing in the water chamber die is initially only a few bar and then rises to a few hundred bar in the finishing shaping process of the workpiece.
According to an advantageous development of the invention, the disturbing force is measured by a change in the hold-down force. It is here possible to use the hold-down cylinders as pressure sensors, so that additional measuring means, such as pressure cells, can be dispensed with. This simplifies the structure of the metal forming press quite considerably. Similarly, in an advantageous development of the invention, the change in the hold-down force can be determined by a change in pressure in the respective hold-down cylinders.
The disturbing force is advantageously compensated by locally different closing cylinder forces. Since in the metal forming press according to the invention the closing cylinders can be arranged in variable order to position the same such that they are adapted to the geometry of the workpiece and the arising forces, it is also possible to act on individual ones of the closing cylinders at different pressures. Thus, according to the invention individual closing cylinders can be acted upon by pressure more strongly than other closing cylinders. The elastic deformation of the tool can be compensated such that the desired hold-down forces are maintained. According to the invention the pressure of individual closing cylinders can thus be changed advantageously in different ways.
For the determination of the necessary parameters and for performing an optimum metal forming process it is particularly advantageous when the sum of the closing cylinder forces is constant for a respective phase of a metal forming process. This means that the sum of the individual closing cylinder forces, i.e. the total closing-cylinder force is distributed over individual control circuits of the individual closing cylinders. The distribution may preferably be in percent, so that a value of 100% is obtained for the total closing-cylinder force. This distribution of the total closing-cylinder force in percent takes into account a hold-down force curve as a reference variable which inter alia depends on the hold-down path and the ram travel and the ram force, respectively. The hold-down force curve is measured via the pressure of the hold-down cylinders, multiplied by the active area. For the individual phases of a metal forming operation deviations from said desired value of the hold-down force or from the predetermined hold-down force curve are then corrected according to a control scheme to be still described. It is thus possible to determine, either empirically or mathematically, both the number of the individual closing cylinders and their control circuits as well as their load carrying capacity in percent. For instance, at the beginning of an optimizing process a uniform load carrying capacity may be assumed. Possible flaws in the tool can thus be compensated or avoided by locally changing the hold-down force. Said local changes in the respective local hold-down forces are effected by changes in the individual closing-cylinder forces. When the individual force of a closing cylinder is increased or decreased, the respective force of the remaining closing cylinders will also change in accordance with the predetermined distribution key in percentage. Such a compensation in percentage is preferably carried out fully automatically within the scope of the present invention.
To achieve an optimum metal-forming result, it may be advantageous that the hold-down force that is respectively optimum in time and location is determined at the beginning and that the change in the closing cylinder forces is varied for maintaining said hold-down force. An optimum curve of the hold-down force at locally very different places can thus be realized by means of the press control.
It is particularly advantageous when for each metal forming process the necessary hold-down forces are divided into individual zone-like areas of the workpiece and determined with respect to their respective value and the individual closing cylinder forces are adapted to the respective areas.
For the determination of the necessary hold-down forces the metal forming operation is preferably divided into individual phases, and the hold-down forces are determined for said phases by means of finite element methods both locally and in time and according to the value. The respective closing cylinder forces can thus be applied to be assigned locally and in time to the hold-down forces.
It is particularly advantageous when the closing cylinder forces are chosen to have a value greater than the closing cylinder forces which are each mathematically predetermined locally and in time.
This yields a closing cylinder force that is increased by a few percent in each of the closing cylinders so as to compensate for possible errors in the mathematical predetermination and for tolerances in the workpiece characteristics. At any rate it is ensured that the water chamber die is sufficiently sealed.
To predetermine the arrangement and position of the individual closing cylinders, it may be particularly advantageous when the resulting total force is determined for the individual metal-forming steps with respect to magnitude and three-dimensional position and when the local assignment of the closing cylinders is carried out in response to the respective position of the resulting total force.
Thus, in the procedure according to the invention the ideal force curve is determined for the partial hold-down forces and the water chamber die pressure in response to the travel of the ram tool by means of finite-element methods and/or by means of computer simulation. The metal forming process is here divided into individual phases and assigned to the respective force curves. The travels (strokes) and the associated force curves determine the functional sequence of the metal forming press. At a great drawing depth and at a small force the metal forming operation can e.g. be carried out via the ram tool only; in the case of flat components with a high force possibly via the closing cylinders only. Normally, i.e. at a great drawing depth and at a great force, the metal forming process is carried out via the ram tool and the closing cylinders.
According to the invention the above-mentioned individual phases can form either time intervals of the forming process or path segments of the ram tool. It is thus possible within the scope of the invention to optimize the forming process in many ways so that it is adapted to the respective requirements. Thus it is e.g. possible to carry out a weighting operation during the stroke of e.g. the ram tool in accordance with predetermined paths (e.g. 1 mm, 1.5 mm, 2 mm, etc.) and to adjust the closing cylinder forces as described above, either in their total magnitude or in their percentage distribution. This applies analogously to the possibility of realizing the individual phases as time intervals. For instance, it is possible that the control unit requests the respective values in steps of milliseconds and performs a corresponding compensating operation.
After the forces (ram force, water chamber die force and partial hold-down forces) have been determined or predetermined over the whole forming process, the forming operation is carried out according to the invention as follows:
1. The closing cylinders are positioned such that the calculated hold-down forces and the closing forces can be introduced in an optimum way. To this end all forces have to be considered throughout the forming process. In the press of the invention it is possible to control individual closing cylinders separately and to switch them on or off, optionally also separately. A switching to a rapid movement is also possible, namely through connection of the annular chamber side to the bottom side. It is thereby ensured that the position of the closing cylinders can be chosen such that the flange movement of the workpiece is locally influenced in an optimum way. The pressure range during the forming process can be chosen such that an optimum result is achieved. It goes without saying that the arising forces can be determined mathematically with respect to their minimum and maximum values to determine both the size and the position of the individual closing cylinders.
After these steps of the process have been completed, the press is closed; the hold-down locks can be retracted. Thus the hold-down cylinders are extended until abutment; the closing cylinders are lowered.
2. For producing the hold-down forces the closing cylinder forces must overcome, apart from the hold-down force, the press and tool parts to be lifted and the resulting forces, the frictional force on the guides and in the cylinders, and the ram force. The latter, in turn, is a function of the water chamber die pressure, multiplied by the actual contact surface between ram tool and sheet metal plate or blank. A further component of the disturbing force is due to an oblique movement of the plate and also has an effect on the water chamber die. The respective hold-down force is directly obtained by sensing the pressures (bottom and annular chamber side) at the hold-down cylinders, multiplied by the active areas. The weights of the hold-down rings and the tool are added to said value. This yields the total hold-down force.
3. A look at the following five phases or steps of the forming process reveals the following relationships for the cooperation of hold-down force and closing cylinder force:
a. close press and lock hold-down device
b. build-up hold-down pressure and pre-arching
c. move down and lock ram
d. move water chamber die and sheet holder upwards by means of closing cylinders, thereby preloading the same against the hold-down cylinders
e. form the workpiece
f. decompress, unlock and open the metal forming press.
Regarding a: After the metal forming press has been closed, the hold-down locks are retracted, the hold-down cylinders are extended until abutment, the closing cylinders are lowered.
Regarding b: The extended hold-down cylinders are blocked and produce a maximum force; the closing cylinders open and clamp the workpiece firmly between the water chamber die and the hold-down ring, so that no plate movement of the workpiece takes place during the preforming operation and the maximum hold-down force is not reached. Fluid is now fed into the water chamber die and is measured either via the pressure prevailing in the water chamber die or via the amount of the supplied fluid until the desired pre-arching is achieved.
Regarding c: The closing cylinders lower their force such that the respectively desired hold-down pressure is achieved in part without retraction of the hold-down cylinders. The hold-down cylinders are rendered pressureless for a short period of time for subsequent use as a xe2x80x9cpressure cellxe2x80x9d. The closing cylinders then move upwards, thereby developing the closing force assigned to them. The sum of the closing cylinder forces must yield the predetermined total hold-down force. For controlling the disturbing forces, such as ram force, friction and weights, the actual hold-down force is determined in a computer. The latter compares the desired value and the actual value and controls the individual closing cylinder control circuits in percent. It is thus ensured that the partial allocation of the hold-down forces takes place according to the predetermined values in the intended ratio. The hold-down cylinders act as rigid spacers and just assume a measuring function.
Regarding d: After the ram or the ram tool has reached its lower dead center and has been locked, the forming process is continued. The clamped workpiece (plate or blank) is now drawn over the fixed ram tool. The hold-down cylinders must apply the predetermined hold-down force. This is accomplished in that pressures depending on the ram path are predetermined for the hold-down cylinders. The pressure curve is monitored by the control unit. In case of deviations these are assigned to the individual closing cylinder circuits to produce pressure compensation in part by the closing cylinders. This, however, would just produce a force equilibrium, but it would not be possible yet to apply a movement. Therefore, the control unit automatically adds an offset to the predetermined values of the closing cylinder forces, with the offset being always a few percent higher than is achievable forcewise. Forcewise achievability means here the production of the predetermined hold-down force and thus the displacement of the hold-down cylinders that corresponds to a further closing cylinder stroke. This method ensures variable hold-down forces with a partially different predetermined distribution in the flange area of the workpiece during the forming stroke.
Regarding e: After the closing cylinder stroke has been completed, the workpiece is in its formed state; however, for producing radii and sharp-edged contours both the closing pressure of the closing cylinders and the water chamber die pressure are raised to a maximum value. A possibly necessary sheet movement of the workpiece is still possible without the sealing effect being overcome between the workpiece and the water chamber die.
Regarding f: After all of the pressures have been reduced, the closing cylinders are lowered, both the ram tool and the hold-down cylinders are unlocked, and the tool is opened.
Thus according to the invention this enables the operator who designs or programs or operates the metal forming press to take into account many parameters for optimizing the forming process. The most important parameters are listed below:
hold-down force curve in dependence upon the position of the ram hold-down device,
curve of the water chamber die pressure in dependence upon the position of the ram hold-down device,
number and position of the individual closing cylinders and/or
number and geometry of the closing cylinder circuits and their pressure curves in dependence upon the position of the ram hold-down device.